It is proposed to investigate the endocrine aspects of calcium homeostasis along the following lines: (1) A study of the regulation of the renal-vitamin D endocrine system in the Japanese quail, particularly with respect to the role of estradiol, parathyroid hormone, and prolactin; (2) Attempt to overcome the problems of working with rat kidney for studies of in vitro metabolism of 25-hydroxyvitamin D3; (3) Complete a survey of the plasma estradiol levels in the Japanese quail; and (4) To establish the methodology for determining plasma testosterone and initiate a survey of the plasma testosterones in the Japanese quail.